The invention concerns a method for conditioning and homogenizing a continuously flowing glass stream in a conditioning stretch, which extends from the inlet side to at least one outlet and at the beginning of which there is a cooling zone, followed by a homogenizing zone for the glass temperature, whereby the temperature in the conditioning stretch is reduced from the inlet temperature T1 to the working temperature T2, preferably for the production of molded glass articles such as containers and pressed glass articles.
Whereas the temperatures necessary for melting glass depend on the con,position, on the production process an on other factors, the temperatures required for processing the glass are normally lower than the melting temperatures of the glass. Consequently the glass must be cooled between the melting and working processes. Cooling of the glass s a part of the so-called "conditioning", during which the glass is prepared for processing. The achievement of the level of thermal homogeneity necessary for the particular working process is also part of the conditioning of the glass.
Conditioning of the glass can only take place when the glass has left the actual melting unit. In the past, the conditioning was mainly carried out in the so-called forehearths or feeders. Nowadays the so-called working end or distribution channel are also used for conditioning.
Certain developments in the recent past have radically changed the situation concerning the cooling of glass. Various improvements have been made in the melting furnaces which have resulted in a significant increase in the specific melting capacity, i.e. the melting capacity related to the area of the melting zone. Consequently the temperature of the glass leaving the furnace has increased. Other melting aids, such as bubblers or bottom heating, which have the effect of increasing the glass temperature on the bottom of the melting tank, have also led to an increase in the temperature of the glass leaving the melting tank.
Continual improvements have also made to glass processing machines, amongst other things to increase the throughput. Whereas in the 1960's and 1970's machines for the mass production of containers were equipped with 6, 8 or 10 stations each for two gobs, nowadays 12 to 16 stations each for two gobs or ten stations each for three or four gobs are used. The throughput capacity of individual machines has therefore been greatly increased.
As a result of the factors mentioned above, significantly more heat must now be removed from the glass after it has left the melting tank and before it is worked than in the past. The increase in the throughput of the individual machines has also reduced the residence time of the glass in those parts of the system where the glass conditioning takes place. Thus, a greater amount of heat must be removed in a shorter time. This results in the fact that the productivity of the complete production line depends to a large extent on the cooling capacity along the conditioning stretch. However, numerous technical problems must also be taken into consideration.
As a result of the relatively high viscosity of the glass, the flow of glass in working ends and forehearths, the basic form of which is normally a channel, is laminar. It is usual for a velocity profile to be established in the glass bath, in which the maximum lies approximately in the center of the flow channel on the glass surface. Since the viscosity depends on the temperature of the glass, there is an interaction between the glass temperature, the heat losses and velocity of the glass. Wherever the velocity in a particular area is lower, the resulting increase in the residence time leads to higher heat losses. Thus the temperature sinks even further, and the increased viscosity leads to an additional decrease in the velocity.
At a constant throughput a reduction of the velocity in one area automatically leads to an increase in the velocity in other areas with higher glass temperatures. This results in a reduction of the residence time in the higher temperature areas and so reduces the effective cooling capacity. For this reason the area of the glass bath affected by a cooling system must be clearly defined, and, as far as possible this cooling area must avoid areas in which there are low flow velocities.
Areas of low temperatures and higher viscosity produce an effective reduction in the flow cross-section, which in turn leads to an increased drop in the glass level between the melting tank and the extraction point. This can also result in production disturbances.
Furthermore, when glass of a certain composition is cooled below a specific temperature limit crystals can be formed, a process known as "devitrification". This process can also cause significant disturbance in the production. Therefore the cooling of the glass bath to temperatures below the devitrification temperature should be avoided. As crystal formation depends on both the temperature and time, the residence time of the glass in the critical temperature range is also an important factor.
The transport of heat within the glass bath itself is almost completely by radiation, whereby the transport velocity depends on the glass composition. For example, the presence of ferrous iron or chromium, which are used as coloring agents in green glass, reduces the rate of heat transport in the glass bath in comparison with a colorless glass. This results in a delay in the heat transport from the lower areas of the glass bath. If the cooling is applied too late, then no effective cooling effect can be observed in the lower areas of the glass bath before the glass reaches the extraction point.
Numerous cooling systems for glass conditioning are known, in most of which the heat transport is primarily by radiation. This type of heat removal is advantageous because the heat is not removed directly from the glass surface, but from a layer of the glass bath, the thickness of which depends on the radiation transmission of the glass. The Stefan-Boltzmann Law is used to calculate the amount of heat transported by radiation. An important factor in this mathematical function is the temperature difference between the radiator and receiver. Applying this function to the case being considered here, the temperature of the radiator is the temperature of the glass, which is determined by the operating conditions of the installation, and, as such, is not a variable in mathematical terms. Therefore the temperature of the receiver determines the amount of heat which is removed.
German patent DE-PS 24 10 763 teaches that the roof of a forehearth channel can be shaped so that a channel, open at the bottom is formed along the center of the forehearth. A stream of cooling air can be passed longitudinally along this channel between the forehearth roof and the glass bath. This produces a temperature difference between the glass in the forehearth and the lower surface of the roof, so that the glass radiates heat to the cooled area of the roof. It is advantageous here that the roof, acting as the radiation absorber, is cooled directly by the cooling air. However, the amount of energy transmitted is determined by the temperature to which the roof can be cooled. In practice the lower surface of the roof cannot be lower than several hundred.degree. C., which limits the actual cooling capacity available per unit length of forehearth. The effective temperature of the radiation absorber, and so the amount of heat which is removed, is varied by varying the amount of air passing along the channel. The cooling air entering the channel is much colder than the inside of the channel, so that the air heats up quickly. If a large amount of air is introduced however, density differences may occur between the colder air and the surroundings, so that convective air movements take place. This leads to the risk of direct cooling of the glass surface by the cooling air, which will result in an acceleration of the hot glass stream below the surface, and consequently to a reduction in the effective cooling capacity of the system.
U.S. Pat. No. 3,582,310 describes an apparatus in which the cooling air channel is separated from the glass melt by means of an intermediate cover, to prevent direct cooling of the glass surface. However the cooling efficiency is also reduced by this cover, as a temperature gradient is established in the intermediate cover, which leads to a higher temperature on the lower side of the cover. This again has a limiting effect on the rate of heat removal from the glass.
A more effective cooling system is described in European Patent 0 212 539. Openings are made in the roof of the conditioning stretch, the effective area of which can be varied by means of sliding tiles. In this way the surroundings are used as a radiation receiver, with the rate of heat transferred being determined by the effective area of the openings. Even in the worst case the temperature of the surroundings is under 100.degree. C., and is therefore much lower than the temperatures which can be reached by radiation receivers in other systems. The cooling capacity per unit area is therefore much higher with this system. However, the radiation openings create a chimney effect and therefore cause convective air movement. Such movements are difficult to control and can lead to control problems.
Even when the cooling capacity is basically sufficient, there may still be problems with cooling the lower layers of the glass bath, which retain too high a temperature, particularly in colored glasses.
European patent application EP-OS 2 195 598 teaches that cooling air can be blown through holes in plates directly onto the glass surface. If a high cooling capacity is necessary, which is the case when the glass throughput is increased, a sort of skin forms on the glass surface, below which glass with a higher temperature and lower viscosity continues to flow at a higher velocity.
U.S. Pat. No. 3,645,712 describes an apparatus comprising a row of plate shaped heat exchangers which can be installed along the complete length of a feeder or forehearth. A cooling medium flows through these heat exchangers, which are not immersed in the glass bath. This cooling can be made very effective by the use of the appropriate cooling fluid, but the temperature only sinks slowly along the whole length of the forehearth, so that the remaining channel length is not sufficient to achieve the necessary homogenization of the temperature over the whole depth of the glass bath. Although the bottom is raised just after the glass enters the forehearth, this publication still recommends the use of a considerable depth in the glass bath.
U.S. Pat. No. 4,029,488 describes cooling units installed in the bottom of the channel at the forehearth entry, such that the glass flows over the cooling units, which are thereby said to exert an intensive cooling effect. At the beginning of the forehearth two cooling units are installed in line along the center line of the channel, and only afterwards are two more cooling units installed side by side. Therefore a strong cooling effect only takes place at some distance from the forehearth entry. This type of cooling unit extracts heat only from the layer of glass directly in contact with it, which is of necessity on the bottom of the channel. In practice it is difficult to move tills layer, even when stirrers are used. The stirrers cannot be installed deep enough to pick up the cold bottom layer as this would cause too much corrosion to the refractories. Furthermore the depth of glass in the channel is relatively high, so that it is difficult to achieve even a more or less homogeneous temperature distribution.
U.S. Pat. No. 2,394,893 teaches the use of a rake-like, cooled stirrer to systematically stir up the contents of a working end. This solution requires a complicated apparatus, and still does not achieve homogenization of the temperature distribution, as there is not sufficient distance available for temperature equalization at the different outlets of the working end.
German Patent DE-PS 25 07 015 describes the use of water cooled stirrers in the melting tank itself, between a melting and refining section with a high temperature on the one hand and a refining zone with a lower temperature on the other hand, in order to increase the homogenization and to improve the quality of the glass. However this requires a longer melting tank, and the problems connected with further cooling and temperature homogenization before the processing of the glass are not solved.
Finally, it is known from German Patent Application DE-OS 31 19 816 that it is possible to divide a forehearth into five zones, the first two of which are a rapid cooling zone and a fine cooling zone. The glass is mechanically stirred in the third zone, and the fourth zone is an equalizing section for homogenizing the temperature before the glass enters the fifth zone, in which the normal gob is formed. Enclosed channels are provided in the roof and the bottom of both the rapid cooling and fine cooling zones for the selective or simultaneous flow of a cooling fluid. The heat removal per unit length of the two cooling zones is, however, still limited, so that the glass flows through a zigzag-shaped channel, in which additional electrodes are installed to heat the glass in the so-called "dead corners". Cooling and additional heating of the glass must therefore be carried out simultaneously, so that large quantities of heat are passed from the additional heating zones to the cooling zones.